Magnetic resonance imaging (MRI) is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field. When a human body, or part of a human body, is placed in the magnetic field, the nuclear spins associated with the hydrogen nuclei in tissue water become polarized, wherein the magnetic moments associated with these spins become preferentially aligned along the direction of the magnetic field, resulting in a small net tissue magnetization along that axis. MRI systems also include gradient coils that produce smaller amplitude, spatially-varying magnetic fields with orthogonal axes to spatially encode the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are then used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei, which add energy to the nuclear spin system. As the nuclear spins relax back to their rest energy state, they release the absorbed energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
As mentioned, RF coils are used in MRI systems to transmit RF excitation signals and to receive MR signals, the RF signals emitted by an imaging subject. Various types of RF coils may be used in an MRI system such as a whole-body RF coil and RF surface (or local) coils. While whole-body RF coils are used in transmit and receive mode, RF surface (or local) coils can be designed either for use in transmit and receive mode or for receive mode only. An operator of the MRI system may select a particular RF coil for a given imaging objective, and so the RF coils are typically removable from the MRI system. It is possible that an RF coil may be unplugged from the MRI system but not removed from the imaging region where the magnetic fields are generated.
Unplugged RF coils in the magnet bore are currently not detected by the MRI system. Typically, RF coils are configured to passively and/or actively decouple the coil from the MRI system in order prevent or reduce absorption of RF energy by the coil when it is not in use. However, unplugged RF coils typically have limited passive decoupling and thus may absorb RF energy during imaging. As a result, unplugged RF coils are prone to producing excessive amounts of heat which can damage or destroy the coil and which poses a safety hazard to the patient who is in contact with the coil. Additionally, unplugged coils can cause image artifacts by distorting local transmit and receive sensitivities of other nearby RF coils used for imaging. In any case, the presence of unplugged RF coils in the MRI system during imaging is typically unintentional and should be avoided.
One approach to alerting an operator of the presence of RF coils is to attach RFID tags to RF coils. However, this approach detects an RF coil based on proximity only, and does not distinguish between RF coils inside or outside the magnet bore.